The invention relates to a process for producing magnetic materials, to new and improved materials produced thereby and to the use of these materials to make permanent magnets.
Magnets have many applications in engineering and science as components of apparatus such as electric motors, electric generators, focussing elements, lifting mechanisms, locks, levitation devices, anti-friction mounts and so on. In order for a magnetic material to be useful for making a permanent magnet three intrinsic properties are of critical importance. These are the Curie temperature (Tc) i.e. the temperature at which a permanent magnet loses its magnetism, the spontaneous magnetic moment per unit volume (M.sub.s) and the easy uniaxial anisotropy conventionally represented by an anisotropy field B.sub.a. The Curie temperature is of particular significance because it dictates the temperature below which apparatus containing the magnet must be operated.
During this century much research has been directed to developing magnetic materials which combine high Curie temperatures and improved magnetic moments with strong uniaxial anisotropy. For many years magnetic materials of the AlNiCo type were used in permanent magnets for practical applications. In the late 1960's it was discovered that alloys of the rare earth elements, particularly samarium when alloyed with cobalt, had magnetic properties which made them superior as permanent magnets to the AlNiCo type. Compounds of samarium and cobalt provided magnets which were particularly successful in many demanding practical applications requiring a magnet with a high energy product. However the high cost of cobalt as a raw material led investigators in the early 1980's to consider the possibility of combining the cheaper and more abundant iron with the magnetically superior rare earth elements to produce permanent magnets with improved magnetic properties. A major breakthrough came in 1983 when the Sumitomo Special Metals Company. and General Motors of America independently developed a magnetic material which combined a rare earth element and iron and incorporated a third element, boron, into the crystal lattice to give an intermetallic compound, Nd.sub.2 Fe.sub.14 B which can be used to produce magnets with an excellent energy product, but a lower Curie temperature than the Sm-Co materials. These Nd-Fe-B magnetic materials can have a Curie temperature of up to 320.degree. C. and are particularly described in three European applications, EP-A-0101552, EP-A-0106948 and EP-A-0108474. Derivatives of these boride materials represent the state of the art to date in magnet technology. However they are somewhat unstable in air and change chemically, gradually losing their magnetic properties so that despite Curie temperatures in excess of 300.degree. C. in practice they are not suitable for operating at temperatures greater than 150.degree. C.
The fact that the incorporation of boron into the crystal lattice of intermetallic materials containing a rare earth element and iron serves to improve magnetic properties has encouraged investigators to search for new compounds of elements other than boron in combination with rare earth elements and iron.
In EP-A-0320064 hard magnetic materials are described containing neodymium and iron but having carbon incorporated to give compounds of the formula Nd.sub.2 Fe.sub.14 C having a similar crystal structure to the known boride materials. In EP-A-0334445 variations of the above type of material having carbon incorporated are described in which neodymium is replaced with praseodymium, cerium or lanthanum and the iron is partly substituted with manganese. Finally EP-A-0397264 describes compounds of the formula RE.sub.2 Fe.sub.17 C where RE is a combination of rare earth elements of which at least 70% must be samarium. The preferred compound described in the last of the above three patent applications, which has carbon interstitially incorporated into a Sm.sub.2 Fe.sub.17 crystal lattice, demonstrates improved Curie temperatures and uniaxial magnetic anisotropy. However it is produced by arc melting of the constituent elements to obtain a casting which is then subjected to an annealing treatment at very high temperatures (900.degree.-1100.degree. C.) in an inert gas. Using such a process puts a limitation on the amount of additional elements which can be interstitially incorporated.
A process for bringing about interstitial incorporation of an element of group VA of the Chemical Abstract Service (CAS) Periodic Table (all references made herein to the "Periodic Table" are being made to the CAS Periodic Table) into intermetallic compounds containing one or more rare earth elements and iron has already been developed by the present inventors and is described in the Applicants' co-pending European Patent Application No 91303442.7 which process comprises heating the intermetallic starting material in a gas containing the group VA element in the substantial absence of oxygen.